Typical data transmission speeds have advanced from only a few to tens of gigabits per second per integrated circuit pin and are expected to further increase as a result of recent advancements in communication and process technologies, as well as system architectures. Serial interface devices, rather than parallel interface devices, are generally used in such high-speed communications. This is because the maximum transmission distance and speed of parallel interface devices are limited due to crosstalk, inductive and capacitive noise coupling, parallel data skew, and the like between parallel conductors. By contrast, serial interface devices convert parallel data into serial data and then transmit differential serial data over a pair of conductors resulting in a small differential signal swing, a process often referred to as differential serial communication. A serial communication receiver receives high speed serial data and converts the serial data into lower speed parallel data.
Unlike parallel interface devices, which transmit a clock signal and data on a parallel link, serial interface devices may transmit data signals without an accompanying clock. Thus, a transmitter encodes the parallel data into serial data containing partial clock information and then transmits them across the link. Meanwhile, the receiver receives the data and then samples and recovers a clock signal and the data from the received data. A clock and data recovery apparatus performs the phase/frequency reconstruction of the clock signal and data recovery from the high speed encoded data signal containing the partial clock information.
However, the pair of conductors used for differential serial communication may be affected by their relative physical length, and by differential-mode interference from other communication devices. As a result, the serial differential communication link may suffer from imbalances that could cause a difference in propagation delay between the pair of conductors, thus causing a phase skew between the differential signals on the serial differential communication link. The phase skew between the differential serial signals is referred to herein as intrapair skew. Intrapair skew refers to a phase shifting within a pair of differential signals on the serial differential communication link. If the intrapair skew is severe enough, then reception of the differential signals becomes difficult or impossible, resulting in errors in the received data. Furthermore, intrapair skew may cause deterministic jitter, also resulting in an increased probability of errors in the received data. The conductor length mismatch, resulting in phase mismatch, is more likely to occur with longer cables. Operations at even higher frequencies further worsens these problems due to reduction of bit time, also resulting in increased bit error rate.
According to one method, compensation for inter-pair skew between differential serial communication links may be performed in order to reduce skew between two or more pairs of differential signals. The difference between inter-pair skew and intrapair skew is that the former refers to as the skew between multiple differential pairs, whereas the latter refers to the skew within a differential pair. A first-in, first-out (FIFO) elastic buffer receives signals from multiple differential serial communication links. The FIFO elastic buffer then timeshifts data received from the various differential serial communication links in an effort to compensate for the inter-pair skew between the serial communication differential links. However, these methods do not compensate for “intrapair skew” between the differential signals on a single differential serial communication link.
It is known to the inventors, intrapair skew reduction may be achieved through the use of a higher-grade cable, such as, for example, a cable having a conductor with a larger cross-sectional area, or with cables that are precise length-matched, using expensive manufacturing equipment. However, these methods introduce higher costs to the production of transmission cables. Furthermore, delay-matching transmission cables requires a high level of manufacturing control to perform. In addition, dynamic disturbances such as interference may not be compensated due to the static nature of the cable.
It is known to the inventors, the skew within a pair of differential signals, such as in a universal serial bus (USB), is continuously adjusted during data transmission if the skew is less than a prescribed value. If the skew is more than the prescribed value, then the method buffers the differential signals as opposed to performing skew compensation. However, methods that attempt to adjust skew during data transmission, for example, on-the-fly, may result in data transmission errors due to reduction of sampling margin for a given instance. In conclusion, continual adjustments attempting to correct intrapair skew during data transmission would increase the bit error rate (BER), especially during high speed data transmission.